Atomic frequency standards generate and maintain a standard frequency output by using the transition between two well-defined energy levels of an atom and the associated constant frequency to control the frequency of a frequency generating means. The atomic transition between two energy levels is employed as a highly stable frequency reference to which the frequency of a variable frequency oscillator, such as a voltage controlled oscillator (VCO) can be electronically locked. The high stability and relative insensitivity to environmental perturbations associated with an atomic reference frequency is transferred to the variable frequency oscillator.
Hydrogen, cesium and rubidium frequency standards have been used to provide atomic controlled oscillators in which the generated standard frequency is usually 5 megahertz (MHz) or 10 MHz. Such frequency standards have usually employed a quartz crystal oscillator controlled by a physics package and associated electronics in an effort to maintain an accurate and stable standard frequency on a long-term basis. The physics package and associated electronics have been used to slave the quartz crystal oscillator to the frequency of the atomic transition thereby reducing the tendency of the quartz crystal to exhibit drifting due to aging and environmental effects.
Such atomic frequency standards have in the past generally been characterized by means to dissociate the atoms, and means to form the dissociated atoms into narrow beams of atoms with a specific energy level with said beams being contained within vacuum systems to remove gasses that might interfere with the beam of atoms. Various electrical and thermal components have been associated with the dissociator and the means to provide the atomic beam. While these various components and elements of the prior atomic frequency standards have been necessary to the operation of the frequency standard, they have introduced long-term and short-term instabilities into the frequency standard. Dissociators, vacuum pumps, beam focusing and atomic separation means and other such components contribute sources of unreliability and increase the size and the weight and the power requirements of atomic frequency standards.
The life of commercial cesium atomic beam frequency standards is limited in part from the use of state selection magnets. Because the atomic state selection is velocity dependent, the magnetic beam optics select a relatively narrow velocity distribution of atoms, around 10 percent of the available velocity distribution. In addition, magnetic state selection works by rejection of unwanted atomic states and accepts only one of sixteen states. Thus, the useful atomic flux is less than 1 percent of the total atomic flux. Consequently, a larger cesium consumption is required for a given clock stability than if all of the atoms were used, contributing to beam tube failure in some cases.
In the microwave interaction region of the standard, the magnetic field (called the "C-field") must be uniform, parallel to the magnetic vector of the microwave field, and stable in time. Failure to make a completely smooth transition from the high field regions of the state selection magnets to the C-field region can produce Majorana transitions which can produce large frequency shifts.
Another limitation to existing cesium beam standards occurs because the velocity distributions of magnetic field dependent transitions (mf=.+-.1) which are adjacent to the clock transition are not symmetrical. This situation, coupled with the detailed lineshape of the Rabi transitions derived from the narrow velocity distribution, produces frequency shifts at lower values of C-field. These shifts contribute to the long term instability of the cesium standard. Increasing the C-field magnitude reduces this effect, but the permissible C-field magnitude is limited by magnetic field instabilities. Operation at certain special C-field values may also reduce the effect.
To avoid these problems, workers in the field have experimented with optical pumping of a beam of atomic cesium and optical state selection to replace state selection magnets. A laboratory cesium beam frequency standard using laser diode optical pumping was reported in 1980, and with further development twenty percent atomatic utilization was reported in 1983 from the use of a single pump frequency derived from a commercially available laser diode. By using two laser diode pumps simultaneously, the atomic utilization was reported to have been increased to forty percent and then nearly one hundred percent. In addition, a three-fold stability improvement in commercial cesium beam standards was reported to result from the replacement of magnetic state selection with optical pumping and fluorescence detection.
There have been other efforts in the art to develop reliable atomic frequency standards having short-term and long-term stability in their frequency output, and such improved reliability and reduced size, weight and power requirements as to permit their economical use and transportation. The long and continuous effort to develop improved frequency standards is exemplified by U.S. Pat. Nos. 3,397,310, 3,403,349, 3,536,993, 3,577,069, 3,718,868, 3,967,115, 4,059,813, 4,354,108, 3,442,658, 4,596,962, 4,684,900, 4,814,728, 4,454,482, 4,425,653, 4,323,860, 4,315,224, 3,670,171, 3,667,066, 3,513,381, 3,495,161 and 435,369. Many of these, such as U. S. Pat. Nos. 3,667,666 and 3,670,171, show a Ramsey microwave resonance cavity, which is a preferred cavity for producing resonances, but which, up to now, has always included two widely separated arms, which has caused standards using such cavities to be relatively large. One patent, U.S. Pat. No. 4,684,900, discloses the use of a laser tuned to the F=4.+-.F'=5 transition, but only as a detection laser, and not as an optically pumping laser. None of the systems of the prior art employ a dielectrically loaded TE.sub.011 resonance cavity in a cesium beam frequency standard.
As set forth above, while the concept and benefits of an optically pumped cesium beam frequency standard have been demonstrated in the laboratory, a practical commercial cesium beam standard has not yet been developed.